1. Field of the Invention
The present invention relates to a secondary surveillance radar and a method of analyzing replies for secondary surveillance radar, which receives replies corresponding respectively to interrogations transmitted to a transponder of an aircraft, and which analyzes contents of the replies.
2. Description of the Related Art
Radars for aircraft surveillance which are used for air traffic control are broadly divided into a primary surveillance radar (PSR) and a secondary surveillance radar (SSR).
The PSR is designed to acquire information on a position of an aircraft by sending out radio waves from the ground, and by receiving and processing reflection waves thereof.
On the other hand, the SSR is designed to obtain various kinds of information on an aircraft as follows. In the SSR, interrogations are transmitted from an interrogator placed on the ground equipment to a transponder installed in an aircraft. Then replies corresponding respectively to the interrogations are received.
The SSR has several modes depending on what kind of information is to be obtained. The modes are categorized into Mode A, Mode C and Mode S. Mode A is used for obtaining identification information on an aircraft, and Mode C is used for obtaining altitude information. Mode S has an individually selective calling function using an ID (a 24-bit address) given to each aircraft. By use of Mode S, route information and velocity information can be obtained in addition to information of the abovementioned kinds (identification information and altitude information) (for example, refer to “Secondary Surveillance Rader for Air Traffic Control—SSR Mode S” (Yoshio Hashida, Hisashi Ootomo and Yoshinori Kuji, Toshiba Review Vol. 59 No. 2 (2004), pp. 58-61)).
Note that, in the following description, “Mode A” and “Mode C” mentioned above will be collectively called as Mode A/C as appropriate.
The abovementioned Mode S and Mode A/C can be used, in the same SSR, in combination with each other. For this reason, even when a Mode S transponder has made an inappropriate reply for some reason, a position and the like of an aircraft can be identified by receiving a Mode A/C reply.
However, in the case of using Mode S and Mode A/C in combination with each other, an interrogator used therefor constantly receives both Mode S reply and Mode A/C reply. For this reason, such a problem as described below sometimes occurs.
That is, when a Mode S transponder is functioning properly, the interrogator can identify an aircraft only with a Mode S reply. However, even in this case, this interrogator receives not only a Mode S reply but also an unnecessary Mode A/C reply from the same aircraft. Then, according to these signals, target reports with respect to Mode S and Mode A/C are generated respectively.
As a method of avoiding a situation like this, a method can be cited in which a combiner is provided in an interrogator for eliminating unnecessary Mode A/C target reports.
However, in the abovementioned combiner, there are problems as described below.
In a general combiner, positional information, an altitude and a Mode A code contained in one target report are compared respectively with those contained in another target report comparable thereto. If these coincide with them under a given condition, the Mode S reply and the Mode A/C reply are judged as having been transmitted from the same aircraft.
However, in a case where a Mode A validity (Va) and a Mode C validity (Vc) in the Mode A/C target report are low (zero), this Mode A/C target report may possibly have been transmitted from another aircraft, and thus cannot be rejected (eliminated).
Additionally, an incorrect collision alert may possibly be issued on the basis of the Mode A/C target report that has not been rejected.
Moreover, a Mode A/C target report, which should not be rejected by right, may be rejected in the following case. In order to unfailingly reject unnecessary Mode A/C target reports, any Mode A/C target report having low validities (Va=0 and Vc=0) is rejected only because of existence of positional correlation with a corresponding Mode S target report. However, distance information contained in each Mode A/C target report indicates a slant range (a linear distance from an antenna to each aircraft). For this reason, the rejected report may be possibly a report regarding another aircraft. The report should not have been rejected.